It is well known in the art of electrographic printing and electrophotographic copying to form an electrostatic latent image corresponding to either the original to be copied, or corresponding to digitized data describing an electronically available image.
In electrophotography, an electrostatic latent image is formed by uniformly charging a photoconductive member and image-wise discharging it by an image-wise modulated photo-exposure.
In electrography, an electrostatic latent image is formed by image-wise deposition of electrically charged particles, e.g., from electron beam or ionized gas (plasma), onto a dielectric substrate.
The latent images thus obtained are developed, i.e., converted into visible images by selectively depositing thereon light absorbing particles, referred to as toner particles, which are typically electrically charged.
In magnetography, a latent magnetic image is formed in a magnetizable substrate by a pattern-wise modulated magnetic field. The magnetizable substrate must accept and hold the magnetic field pattern required for toner development, which proceeds with magnetically attractable toner particles.
In toner development of latent electrostatic images two techniques have been applied: “dry” powder development and “liquid” dispersion development. Dry powder development is nowadays most frequently used.
In dry development, the application of dry toner powder to the substrate carrying the latent electrostatic image or magnetic image may be carried out by different methods, including “cascade”, “magnetic brush”, “powder cloud”, “impression,” and “transfer” or “touchdown” development methods. See, e.g., Thomas L. Thourson, IEEE Transactions on Electronic Devices, Vol. ED-19, No. 4, April 1972, pp.495-511.
In liquid development, the toner particles are suspended in an insulative liquid, both constituents forming together the so-called liquid developer. During the development step, the toner particles are deposited image-wise on the latent electrostatic image-bearing carrier or magnetic image-bearing carrier by electrophoresis (under the influence of electrical fields) or magnetophoresis (under the influence of magnetic fields). In these particular development steps, the toner particles have, respectively, an electrical charge or a magnetization.
Whereas liquid toner systems have been commonly employed in the past due to their high performance in terms of resolution and image quality, dry toner systems are currently more popular, as they are capable of achieving similar image quality while offering at the same time the advantage that no solvent emission is involved. Liquid toner compositions and methods of using same are disclosed in copending U.S. application Ser. No. 10/372,645, filed on even date herewith and entitled “LIQUID TONER COMPOSITION.”
The visible image of electrostatically or magnetically attracted toner particles is not permanent and has to be fixed. Fixing is accomplished by causing the toner particles to adhere to the final substrate by softening or fusing them, followed by cooling. Typically, fixing is conducted on essentially porous paper by causing or forcing the softened or fused toner mass to penetrate into the surface irregularities of the paper.
Dry development toners typically comprise a thermoplastic binder including a thermoplastic resin or mixture of resins (see, e.g., U.S. Pat. No. 4,271,249) and coloring matter, e.g., carbon black or finely dispersed pigments. The major challenge with respect to dry toning systems is related to the fusing process. The preference for higher process speeds and for a broad spectrum of final substrates, as well preferences for various thicknesses, pose additional stress on the fusing process. Apart from these considerations, there is also the tendency to prefer smaller particles and thinner toner layers. Whereas it could be expected that thinner toner layers are more easily fused, it is observed in reality that this leads to more pronounced fusing problems. The reason is that higher concentrations of pigments are needed in thin toner layers in order to reach the target optical density. These higher concentrations induce a higher melt viscosity, which results in a marked decrease in fusing performance of such toner particles.
There are different types of processes used for fusing a toner powder image to its final substrate. Some are based primarily on fusing by heat, others are based on softening by solvent vapors, and others by the application of cold flow at high pressure under ambient temperature conditions.
In fusing processes based on heat, two major types of processes are typically employed: “non-contact” fusing processes and “contact” fusing processes. In non-contact fusing processes there is no direct contact of the toner image with a solid heating body. Such processes include, for example: an oven heating process in which heat is applied to the toner image by hot air over a wide portion of the support sheet; and a radiant heating process in which heat is supplied by a light source, e.g., an infrared lamp or flash lamp, which emits infrared and/or visible light that is absorbed by the toner. In such “radiant” non-contact fusing processes, radiation (such as infrared radiation) may be at least partly absorbed by the final support and therefrom transferred by conduction to the toner image(s) deposited thereon.
Non-contact fusing has the advantage that the non-fixed toner image does not undergo any mechanical distortion. The fine image details do not suffer distortion from transfer to a contacting fixing member, the so-called “offset” phenomena typically observed for hot pressure roller fusing. Non-contact fusing, however, has the major disadvantage that in the case of a process malfunction the final substrate or support can remain in the hot fusing zone for an undesirably long time, such that the substrate heats up to ignition temperature, thereby causing a fire hazard. This is especially a risk in the case of cut sheet-based engines. Special, costly measures have to be taken to avoid this major danger. Aside from this disadvantage, there is some difference between colors in fusing quality and image quality of the fused image, as the spectral absorption coefficients are not equal over all colors present in the print.
An alternative to “non-contact” fusing that is commonly employed is the so-called “contact” fusing process. In contact fusing, the support carrying the non-fixed toner image is conveyed through the nip formed by a heating roller (also referred to as a fuser roller) and another roller backing the support and functioning as a pressure-exerting roller (also referred to as a pressure roller). This roller may be heated to some extent so as to avoid strong loss of heat within the copying cycle. Other variations on the contact fusing process include use of a fuser belt combined with a pressure roller, or a combination of a fuser belt and a pressure belt.